In WiMAX systems, several multiple antennae transmit techniques have been adopted to deliver extended capacity and/or coverage in the DL (Down Link), with little or no cost to the mobile station. These MIMO (multiple input multiple output) techniques usually include STC (Space-Time Coding) Matrix A (aka the Alamouti scheme), which is commonly used as a means to provide diversity, and adaptive beam forming algorithms intended to focus transmitted energy on individual subscribers using relatively narrow beams.
Although provisions were made in the 802.16e mobile WiMAX standard to enable the aforementioned approaches when transmitting the individual DL traffic, the possibilities are much more limited for achieving a comparable performance boost when transmitting broadcast DL and UL (Up Link) maps messages. While the adaptive ‘narrow’ beam forming techniques, per definition, cannot provide simultaneous coverage across the entire sector when a broadcast message is transmitted, applying STC Matrix A to these maps messages is precluded by the WiMAX standard and profiles. Thus, even when the individual data connection's range can be extended using the MIMO techniques, the cell coverage won't be extended at all due to the limited coverage during transmission of the maps zone.
One common approach to overcome this bottleneck is using a stronger FEC (Forward Error Correction) (with repetition coding), at the immediate expense of capacity, due to greater overhead of map messages. Another approach is applying the CDD (Cyclic Delay Diversity) technique, used in many technologies, to provide additional diversity, by transmitting from the second antenna a cyclically delayed replica of the signal. CDD is widely recognized to perform well in rich fading environments, but raises many issues in line-of-sight (LOS) or near line-of-sight situations, since it can create interference. Performance degradation due the interference can be reduced by advertising the CDD to the receiver, i.e., signaling to the subscriber explicitly with a dedicated message describing how the CDD scheme is applied in the DL transmitter, with an additional overhead of this signaling, but at the expense of the useful bandwidth. One additional aspect of using either CDD or STC Matrix A techniques is that, while both are well defined for the two transmit antennae case, it could be very challenging to extend their usage beyond this number of transmit antennae available in the base station.
In LTE, the DL control channel is transmitted using the SFBC (Space-Frequency Block Coding) transmit diversity (T×D) scheme using all the available transmit antennas (2 or 4). In practice, when four transmit antennas are available, the T×D precoding scheme dictates using only two of them at each given time instance thus precluding exploiting the full power available in the transmitter.
Accordingly, there is a long felt need for a method and system for extending wireless coverage and/or capacity to all portions of the transmitted traffic, and it would be very desirable to provide this extension by permitting usage of multiple transmit antennae without the conventional bottleneck.